It is a well-known phenomena that underground electrical distribution cables typically include an electrical conductor surrounded by a semi-conducting polymeric shield, which is then jacketed with a polymeric insulation jacket. The polymeric insulation jacket may then be further layered with a semi-conducting insulation shield, and finally, an outer polymeric protective jacket is typically applied over the insulation shield. The conductor may be stranded from multiple wires, or less commonly a solid conductor core may be utilized. It is a well-known phenomena that after such electrical distribution cables are buried in the ground for extended periods of time, the polymeric insulation jacket of the cable may undergo deterioration that reduces its dielectric properties and can lead to failure. This situation, which is particularly prevalent with polyolefin insulations, is referred to as electrochemical tree formation, and is caused by the diffusion of moisture into the polymeric insulation. This process can greatly reduce the useful life of electrical cables.
As a result, techniques have been developed for treating such installed cables with an anti-treeing agent that retards the entry of moisture into the insulation layer. A tree retardant or anti-treeing agent is typically a low-viscosity liquid that can be introduced into the interstitial voids assisting between the strands of a stranded conductor cable, which then diffuses out through the shielding and into the polymeric insulation jacket. Alternately, when a solid conductor is utilized, anti-treeing agents can be injected underneath the outer protective jacket and diffuses inwardly through the insulation jacket. Known techniques for treating cables in this manner are disclosed, for example, in U.S. Pat. Nos. 4,372,998 to Bahder and 5,372,840 to Kleyer et al., disclosures of which are hereby expressly incorporated by reference.
For large diameter cables (&gt;500 kcm or &gt;240 mm.sup.2) with stranded wound or loose conductors, the amount of fluid that can fit in the interstices of the strands may exceed the amount of fluid required to optimally treat polymeric cables. Because these cables all have varying electrical loads in use, they exhibit corresponding resistive energy-induced temperature swings. As the temperature of the polymeric insulation varies, so too does the solubility of fluids (such as anti-treeing treatment fluids residing within the cable core and absorbed into any insulation jacket), and hence a condition of "supersaturation" can occur as the temperature cycles down. The fluid is forced from the polymeric phase of the insulation jacket and into tiny microvoids, which are created by the mechanical pressures resulting from the thermodynamic equilibrium associated with the change of phase from the anti-treeing fluid as it passes from being dissolved in the polymeric solid into a free liquid. During the next increase in temperature still more fluid is drawn into the polymeric phase, and the cycle repeats until the swell of the polymer reaches a point where the mechanical strain bursts the cable and it fails catastrophically.
The failure mechanism described above has been observed by the inventors in two classes of cases. In the first class, 750 kcm feeder cables were treated with an anti-treeing agent sold commercially by Utilx Corporation, Kent, Wash., under the trademark CableCURE 2-2614 (as disclosed in U.S. Pat. No. 4,766,011, issued to Vincent et al., the disclosure of which is hereby expressly incorporated by reference) fluid for a period from 1990 to 1991 at Arizona Public Service (APS). Reservoirs of fluid were left attached for 60 days as this was the standard practice for all cables treated prior to this time frame. The application to large diameter cables was new. A large number of these cables failed in-service due to the supersaturation mechanism described above. The procedure of leaving a pressurized soak bottle attached to cables larger than 3/0 in size was discontinued.
A second class of observations involved an experiment at Cable Technology Laboratories (CTL) undertaken on behalf of Orange & Rockland utilities. A 4/0 (relatively small) diameter cable was thermally cycled with a pressurized reservoir of CableCURE fluid attached. The cable failed as described above. In actual field application, no reservoir is attached to such a cable, so that there has not been a chance for such a failure mechanism if proper procedures are followed. The problem was thought to have been solved by eliminating the external pressurized reservoir.
Until the current unexpected problem, which is the inspiration for the present invention this procedural change solved the problem. While eliminating the pressurized reservoir was and is sufficient for many cables, certain large diameter conductors, especially those with thinner conductor shields and/or thinner insulation have experienced failure due to supersaturation. FIG. 1 is a Cable Field Report (CFI) for such a cable. The 1000 kcm cable was treated on Feb. 2, 1998 and failed on Jul. 30, 1998.
FIG. 2 is a micro-infrared spectrographic analysis of the cable described in FIG. 1, labeled Texas Utilities (TU.) 00023210. Four radial scans quantifying the anti-treeing agent sold commercially by Utilx Corporation under the trademark CableCURE/XL fluid (as disclosed in U.S. Pat. No. 5,372,841, issued to Kleyer et al., the disclosure of which is hereby expressly incorporated by reference) were taken (90.degree. apart from each other and labeled 1.sup.st Quarter through 4.sup.th Quarter) from the conductor shield out to the insulation shield and are plotted. An insert of a well-treated cable labeled OG&E Cable Phase B is provided for comparison. The integrated quantity of fluid in the dielectric of the TU cable is approximately twice that of the OG&E cable.
The dilution of dielectric enhancement fluids (i.e., anti-treeing agents) has been proposed for other purposes. Bertini teaches in U.S. Pat. No. 5,200,234, the disclosure of which is hereby expressly incorporated by reference, that diluents can be used to treat cables from the outside in. This prior art teaches that since there is such a gross oversupply of fluid in the annulus of the conduit contemplated in that disclosed method, that dilution is an economic requirement for outside-in treatment to be feasible. The prior art did not consider supersaturation an issue. The TU failure is an unexpected result of an inside-out injection.